A continuacion una breve introduccion a la STL de C++

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CHAIN of actions. A component of my Master’s thesis will involve translating some MATLAB code to C++. The MATLAB code includes a check for whether an iterative process is taking too long, in which case it quits. So, I wanted to download a simple C++ stopwatch class implementation. I Googled “stopwatch class” , and thought that the Code Project’s stopwatch implementation seemed to fit the bill.

Interestingly, the National Center for Biotechnology Information has their own C++ stopwatch class implementation. I would’ve used it since it was consistent with my bioinformatics theme, but unfortunately the NCBI stopwatch seems to depend on another class called a SmartString, which deterred me; I wanted something self-contained. (Not sure where SmartString comes from. Is that NCBI stuff? Or is it in the Standard Template Library?) My problem was, suppose I went ahead and downloaded the class for a SmartString. I might then be faced with yet another dependency, and have to download other stuff: a recursive chain of dependencies with an uncertain terminus. (Maybe I could use a stopwatch, and if it took too long to download all the recursive dependencies I could quit the process. ) The Code Project’s stopwatch implementation, on the other hand, seemed to be short, light-weight, and self-contained; this reduces the variability of whether I can get the code to work or not. Maybe I’ll check out the NCBI code later, but right now my main concern is translating MATLAB code to C++ ASAP.

As I’ve gotten older I’ve come to appreciate the wisdom of the K.I.S.S. Principle.

So I proceeded to download the Code Project’s stopwatch class, and to do this I had to register. And one of the questions upon registration was whether I was currently in Daylight Saving Time. I wasn’t sure! I mean, I know that we set the clock back in the fall, and then forward in the spring (“spring forward, fall back”), but I didn’t know which time of the year is considered to be in “daylight saving.”

So I Googled “daylight saving time” and found this cool web page (hope it works in your browser). I browsed down through the “clouds” (nodes) Choose your region… –> North America –> United States and determined that I am currently in Standard Time, not Daylight Saving Time. Give it a try!

The underlying technology is called SpicyNodes. It is very reminiscent of mind-mapping software, e.g. FreeMind.

Many of us who use STL containers and their iterators may need to delete elements while iterating. An example case is:

Delete the game objects that are dead.

To fulfil the above case, you need to iterate through game objects, check their health and delete them if they are dead. The pseudocode would be:

FOREACH game object in game object list
	IF game object's health is less than or equal to zero
		REMOVE game object from game object list
		DELETE game object

Many of us will write the above pseudocode this way:

std::vector< GameObject * > gameObjectList; ///< Game object list

// Throw in some zombies.
gameObjectList.push_back( new GameObject("Zombie01") );
GameObject * zombie02 = new GameObject("Zombie02");
zombie02->kill(); // Kill the zombie02.
gameObjectList.push_back( zombie02 );
gameObjectList.push_back( new GameObject("Zombie03") );
gameObjectList.push_back( new GameObject("Zombie04") );

// Initialize the the iterator.
std::vector< GameObject * >::iterator iEnd = gameObjectList.end();
std::vector< GameObject * >::iterator i = gameObjectList.begin();

while ( i != iEnd )
{
	GameObject * gameObject = * i;
	if ( gameObject.isDead() )
	{
		gameObjectList.erase( i );
		delete gameObject;
	}
	i++;
}

The above code seems perfectly fine but when executed you will notice that after deleting zombie02, the iterator will be invalidated and crash at i++. This is because i is a reference to the element previously storing zombie02 which is now removed. The solution is to make i set to the next iterator after zombie02 (which in this case is zombie03). STL containers erase function will return the next iterator after the one specified in its parameter. So the correct code would look like this now.

while ( i != iEnd )
{
	GameObject * gameObject = * i;
	if ( gameObject.isDead() )
	{
		i = gameObjectList.erase( i );
		delete gameObject;
	}
	else
	{
		i++;
	}
}

P.S. I may update this post with more solutions when I have the solution and time.

Do tej pory AGE było tylko biblioteką statyczną. Zechciałem więc żeby dało się skompilować jako DLL. Dowiedziałem się co trzeba zrobić, wrzuciłem odpowiednie kawałki kodu tam gdzie trzeba. Kompilacja. Linkowanie. Wszystko poszło OK. Mam age.dll oraz age.lib więc zabieram się za test.

Tworzę sobie nowy projekcik piszę to co trzeba i uruchamiam. Działa! Pokazało mi się puste czarne okienko – i o to chodziło ^^ Problem pojawił się kiedy stworzyłem klasę dziedziczącą po ageTask. Nagle wyskoczył błąd i program się wywalił.

Skompilowałem sobie więc age_d.dll (debug) i spróbowałem raz jeszcze. Krótka randka z debuggerem i okazało się, że std::string ma jakieś problemy z alokowaniem pamięci. Poszperałem w necie, zapytałem na forum i moje przypuszczenia się potwierdziły: DLL i STL to nie jest dobre połączenie.

Udało mi się jednak znaleźć stronę na której jest sposób na obejście tego problemu. Okazuje się jednak, że możliwe jest to dla std::string oraz std::vector a co z potrzebnym mi std::map? Nic z tego!

Dlatego też postanowiłem napisać własną mini-bibliotekę zawierającą klasy  list, string oraz map. Klasę string właściwie już skończyłem. Interfejs ma prawie identyczny z std::string. Mówię prawie, bo dodałem do niej metodę print( char *format, … ) dzięki której będzie można formatować wygodnie tekst oraz powycinałem te mniej potrzebne metody ^^.

Jeśli chodzi o list to interfejs będzie zupełnie inny niż ten z std::list lub std::vector. Powodem jest nie mniej nie więcej tylko to, że są po prostu brzydkie ^^.